Styrene distillation



Dec. 28, 1948. T. A. GADWA STYRENE DISTILLATION Filed Sept. 21', 1946INVENJOR. 777L036]! of. fiadma ATTY Patented Dec. 28, 1948 Truman A.Gadwa, to The Lummus 4. Claims.

Mount Vernon, N. Comp corporation of Delawar- Appiication September 21,1946, Serial No.

This invention, an improvement On my Patent No. 2,370,948 issued March6, '1945, relates to the production styrene and more particularly to theseparation of styrene from an exceedingly complex mixture includingcompounds boiling very close to styrene.

In the'production of styrene, it has been found desirable to utilizematerials such as propane and benzene as the principal raw materials.Such propane is appropriately cracked to produce ethylene, which is thenbenzene to produce ethyl benzene. The ethyl benzene is then subjected toa. suitable dehydrogenation treatment for the formation of the desiredstyrene. dehydrogenation step comprises a mixture containing benzene,toluene, unconverted ethyl benzene, styrene, isopropyl benzene, higherboiling substances (high boilers), and a residue of still higher boilingmaterial of an exceedingly complex nature. Distillation of this mixtureof saturated and unsaturated compounds with their mutual solubilities toeffectively separate styrene therefrom is rendered extremely diflicultbecause of the relatively close boiling points of theseverai compoundsand because of the tendency of styrene to rapidly polymerize when it issubjected to heat. It is commonly stated, forexample, that styrenecannot be recovered from a styrene-containing mixture containing ethylbenzene methods.

It is the primary object of my invention to efiiciently obtain styrenefrom a mixture thereoi with other materials whereby styrene ofsubstantial purity is recovered and whereby the remaining materials areavailable for commercial use.

One of the principal objects of this invention is to provide an improvedfractional distillation process for the separation of a syntheticstyrenecontaining mixture to obtain a high yield of styrene having ahigh degree of purity without undue polymerization of the styrene duringthe distillation and a suitable series of other products also of asubstantial purity.

It is a further object of vide for an initial efiicient and economicalseparation of the styrene and the higher boiling compounds from thelower boiling compounds in a fractionating column operated under asubstan;

by usual distillation be hereinafter described .10 alkylated with theany, New York, N. Y., a e

Y., assignor tial vacuum, in a manner and for a purpose to in detail.

in connection with the attached drawing, which is a diagrammatic flowsheet of a stem for Range Example Percent Benzene .64 Toluene 4.8 EthylBenzena. 63. 3 Styrene 20.6 Isopro ylBenzene .3 H1 l1 oilers-.., 4-6 .8Residue 3.7

The mixture, which constitutes distillation system, is passed throughconduit l0 and the heater ll, wherein it is suitably preheated, and isintroduced into the first fractionthe feed to the ating column l2operated at about 160 mm. Hg

my invention to proabsolute pressure. In this column a separationrelatively high-boiling material consisting of the remaining ethylbenzene, the styrene, and the higher boiling materials as bottoms. Theconditions are desirably so maintained that a part of the ethyl benzenetively small portionof the toluene appears in the bottoms.- The overheadis removed through and is condensed in condenser I 5. A part of the intothe third column The bottoms from column I2 is removed through line 20and is fed into the fourth or flash column 22. A portion of the bottomsis desirably circulated by means of line 23 through the reboiler 24 tosupply heat to column I have found. it advantageous to operate column I2under a substantial vacuum as described above. The main result ofoperating at this pressure is that a portion of ethyl benzene is passedor! as overhead in column I2 along with the toluene, benzene, and withonly a trace of styrene in this mixture.

In the actual operation of this distillation system, it has been foundthat by operating column I2 under a vacuum the maximum allowable inputto column I2 is almost double that permissible when it is operated atatmospheric pressure. The reason for this is that when column I2 isoperated at atmospheric pressure nearly all of the ethyl benzene ispassed through line 20 to column 22 and the latter column must beoperated at its capacity. By first removing a portion of the benzene incolumn I2, the load on column 22 is substantially reduced thus allowingan increased input to the system at column I2.

Only one additional change is necessary in this system. Since-column l2does not have enough plates to accomplish the nearly complete separationof styrene from the overhead mixture when above-described manner, moreplates must be added. In actual operation of a similar plant, this isreadily accomplished by interchanging columns I2 and 90. Column so has agreater number of plates which are not needed with the decreased loadresulting from the large amount of vacuum.

' of ethyl benzene being previously removed in column I2.

The optimum pressure to use in column I2 has been found to be about 160mm. Hg absolute. At this pressure over 20% of the total weight of ethylbenzene fed to column I2 is removed in that column as overhead and theremainder passes through line 20 as bottoms to be fed to column 22. Atpressures either lower or higher than about 160 mm. Hg absolute, alesser amount of ethyl benzene passes off asoverhead in column I2.

The relatively low-boiling material is removed from accumulator tank I8and is pumped by means of pump 25 into the second column 26, wherein aseparation is made between the benzene as overhead and the toluene andethyl benzene as bottoms. Conditions in column 28 are desirably somaintained that only a trace of toluene appears in the overhead and onlya trace of benzene and the styrene appear in the bottoms. The overheadis removed through line 21 for condensation in condenser 28, part of thecondensate from which is returned through line 29 as reflux for column26. The remainder of this benzene condensate is removed through line 30and may be recycled to the benzene-ethylene alkylation stage. A portionof the bottoms is desirably circulated by means of line 3| throughreboiler 32 to supply heat to column 26. This column is preferablyoperated at or slightly above atmospheric pressure.

The bottoms from column 26is removed through line 33 and is introducedby means of pump 33a 34, in which a separation between the toluene andthe ethyl benzene is effected. The distillation conditions are desirumnably maintained such that the trace of benzene contained in the feed tothis column and only a trace of ethyl benzene appear in the overhead andonly a trace of toluene and the styrene appear in the bottoms. Thetoluene overhead is removed through line 35 and is condensed incondenser 36, a part of the condensate from which is returned as refluxfor column 3d through line 31. The toluene product is removed throughline 38 and has such a degree of purity that it can be used fornitration purposes. Heat may be supplied to column 34 by circulating aportion of the bottoms therefrom through reboiler 39 by means of line40. The bottoms product comprising primarily ethyl benzene is removedthrough line 42 and may be recirculated to the ethyl benzenedehydrogenation stage.

As previously indicated the bottoms from col- I2, containing styrene andthe rest of the ethyl benzene, is introduced into the fourth column 22through line 20 by means of pump 43. This bottoms stream is preferablysufficiently preheated in heater 45 so that it can be flashed withincolumn 22 to separate the ethyl benzene and the styrene from the higherboiling materials. This flashing operation is so carried out that thetrace of toluene contained in the feed to this column and some of theisopropyl benzene and the high boilers appear in the overhead and tracesof ethyl benzene and styrene and some of the isopropyl benzene and highboilers appear in the bottoms. The bottoms stream containing all theresidue is removed from column 22 through line 46. A portion of thebottoms from this column may be recirculated through heater 45 by meansof line 46a. The ethyl benzene-styrene overhead is removed through line41 for condensation in the condenser 48. A portion of the resultingcondensate is returned through line 49 to column 22 as reflux, and theremainder is conducted to accumulator 50 through line SI. umn 22 isdesirably operated under a vacuum preferably on the order of 50 mm. Hgabsolute pressure, and a vacuum connection 52 may be provided onaccumulator 50 to produce the desired degree of vacuum.

In accordance with my invention, the condensate from accumulator 50 ispumped by means of pump 54 through line 56 into a distillation systemcomprised of a plurality of separate but interrelated distillation zonesfor the purpose of effectively separating the remainder of ethyl benzenefrom the styrene without undesirable polymerization of the styrene. Aspreviously indicated, the charge to this distillation system consistsgenerally of a trace of toluene a part of the ethyl benzene, some of theisopropyl benzene, substantially all of the styrene, and some of thehigh boilers.

Any quantitative separations of the styrene from the ethyl benzene bysimple distillation are usually very diflicult because of theunsaturated nature of styrene, which tends to polymerize at temperaturesand .polymerizes more rapidly with the application of heat even ifinhibitors are used. In addition, the relatively close boiling points ofstyrene and ethyl benzene tend to interfere witha good separationbetween these two materials. According to my invention, it is nowpossible to obtain by distillation a high purity styrene in a relativelyhigh yield and substantially all deleterious polymerization of thestyrene is avoided.

In accordance with a preferred form of embodiment of my invention, theethyl benzenethrough line 8| with answer styrene distillation systemconsists of two distillation zones entirely separated from each other.The feed from accumulator 50 is fed into column 60 wherein a separationis eil'ected between ethyl benzene as the overhead and an ethylbenzenestyrene concentrate as the bottoms. This feed may be introducedat any other point in the distillation system, ii. desired, preferablyin accordance with its composition. Column Ell is provided with bubbledecks, the number of which is selected with respect to the totalpressure drop desired through the column so that the temperaturecorresponding to the effective pressure at the bottom of the column willnot be so high as to cause a substantial degree of undesirablepolymerization of the-styrene.

The overhead from column 60 is removed through line 64 and is condensedin condenser 65, the condensate from which is collected in accumulator85. A portion of this condensate is returned through line Seas refluxfor column 60, and the remainder is removed from the system through line61. The distillation conditions in column 60 are desirably so maintainedthat this ethyl benzene product contains the toluene in the feed to thiscolumn and only a very small portion of the styrene. This ethyl benzeneproduct may be recirculated to the ethyl benzene dehydrogenation stagefor the formation of further styrene. Heat circulating a portion of thebottoms therefrom through reboiler 11 as by means of line 19. Steam maybe introduced into reboiler "through line 18 with the condensate removedthrough line 18a.

The bottoms from column 60 i removed through line 68 and is pumped bymeans of pump 69 through line into the top of column 12, the secondstage of my improved styrene distillation system. This bottoms streamcomprises substantially all the styrene admixed with some of the ethylbenzene, which is separated from the styrene in column 12 by means of astripping action. In column 12 a separation is made between an ethylbenzene-styrene mixture as the overhead and a styrene concentrate as thebottoms. Conditions in column 12 are desirably so maintained that only atrace of ethyl benzene and the isopropyl benzene and pear in the styrenebottoms stream. Column I2 is also provided with bubble decks, the numberof which is so selected that the effective pressure maintained at thebottom of the column corresponds to a temperature which will not eifectundesirable polymerization of the styrene to a of the bottoms fromsubstantial degree. A portion column 12 may be circulated throughreboiler 80 as by means of line 82 to supply heat to the column. Steammay be introduced into reboiler at the condensate removed through line 8la.

The ethyl benzene-styrene overhead from column 12 is removed throughline It and is entirely condensed in condenser E6, the condensate fromwhich is collected in accumulator tank 86, This condensate is passedthrough line Ma to reboiler l7 wherein it is partially or completelyvaporized. The resulting vapors are then introduced into the bottom ofcolumn til.

Columns 69 and 72 are both maintained under a substantial vacuum, whichmay be in the order of 50 mm. Hg absolute pressure. The necessary vacuummay be independently applied to each column, but a common manifold tomay connect the two columns with the vacuum-producing system. Theapplication of a substantially may be supplied to column 60 byethylbenzenetillation system, I was able to reduce the temperature of thestyrene bottoms product removed from column l2 through line 75 byapproximately 20 E; a decrease in styrene polymerization resulted, andan improvement in the fractionation occurred. With the system asdescribed, the pressure drop in each distillation column can be kept ata minimum with a resulting substantially lower overall pressure dropbetween the respective ends of the distillation system. It is,therefore, possible to maintain throughout the distillation system asuillciently low absolute pressure to prevent any substantial degree ofundesirable polymerization of the styrene.

It will be seen that the ethyl benzene-styrene mixture is thus given atwo-stage separation in the respective distillation columns 60 and 72.The maximum difference in pressure between the top and the bottom of therespective columns arising from the pressure drop through themultiplicity of decks therein is not great, however, since each columnis subjected to a vacuum, which arrangement is made possible by theseparate condensahigh boilers aption of the overhead vapors from eachcolumn. Although columns 60 and 12 are thus operated separately, theyare also operated in an interrelated manner in that the overheadcondensate from column 12 is revaporized and the resulting vapors areintroduced into the bottom of column 60, and in that the bottoms fromcolumn 60 is introduced into the top of column 12 wherein it serves asreflux. Without such separate operation of these two columns, thepressure at the bottom of column 12 could not be lowered so as to obtainthe desired reduction in temperature at this point. It is possible undersuch circumstances to obtain a styrene fraction having the .desiredpurity or concentration as the bottoms stream removed from column 12through line H.

Although I have described the separation of the ethyl benzene-styrenemixture as being effected in a two-column distillation system, it willbe appreciated that this operation may also be effectively carried outin a plurality of columns operating in this manner. If it is desirableto do so on an economic basis, this separation process may be carriedout in a single multisection column, in which the sections are separatefrom each other.

The styrene concentrate obtained as the bottoms stream from column 12 isremoved therefrom through line 75 and is pumped by means of pump 89 intothe final distillation column 90. In this column the styrene isseparated from the greater portion of the isopropyl benzene and the highboilers associated therewith. The operating conditions in column 90 areso maintained that a trace of ethyl benzene and a small portion of theisopropyl benzene and the high boilers appear in the styrene overheadand only a very small amountv of styrene appears in the bottoms stream.The styrene overhead is removed through line Ql for condensation incondenser 92.

A portion of the resulting condensate i return-ed through line 93 tocolumn 90 to serve as reflux therefor and the remainder of thecondensate is conducted to the accumulator tank t8,- from eflecting aninitial which the styrene is withdrawn through line Ola.

under a vacuum in the order of 50 mm. Hg ab-' solute pressure in orderthat a sufliciently low temperature may be maintained within column 98to prevent undue polymerization of the styrene. For this purpose, avacuum connection 99 is provided for accumulator 98.

Thedistillation system as above described has certain operatingadvantages which make its application to the, recovery of styrene bydistillation particularly desirable. By effecting a separation in columni2 between the benzene, toluene and a part of the ethyl benzene, and thestyrene, the higher boiling compounds and the remainder of the ethylbenzene, I subject the styrene to the minimum number of heating stepsand to the minimum time or heating whereby the possibility for thestyrene to polymerize because of the heating is materially lessened. Bymaintaining a minimum amount of hold-up liquid on each bubble deck, Inot only obtain efficient fractionation. but I also reduce thepolymerization of the styrene still further since the time during whichthe styrene is subjected to heat is reduced to a minimum thereby.

The operation in the flash fractionation column 22 may also be so variedas to effect the operating conditions in the final column 90. I havefound it desirable, however, to remove overhead from column 22 a portionof the isopropyl benzene and the high boilers so as to reduce the lossof styrene in the bottoms from column 22. In the operation of column 22,I have also found it desirable to effect some degree of fractionation,for which purpose the desired number of fractionating decks may beinstalled therein. It may also be desirable to employ some strippingdecks in the lower portion of column 22 to further reduce the loss ofstyrene in the bottoms-therefrom; in such case the degree of flashing ofthe incoming feed to this column can be considerably reduced. The use ofthis column 22 enables me to eliminate the high boilers and the residuefrom the system before they can interfere with eilicient reboiling ofthe styrene bottoms stream.

Although I have' described a preferred procedure for carrying out myinvention, it will be apparent that modifications may Accordingly, onlysuch limitations as appear in the claims appendedihereinafter should beapplied.

I claim:

1. A step in the process of separating styrene from a styrene-containingmixture derived from the dehydrogenation of ethyl benzene to styrene andincluding benzene, toluene, ethyl benzene and higher boiling materials,without appreciable polymerization of the styrene, which comprisesdistillation under a pressure of about 160 mm. Hg absolute, removingtoluene, lower boiling materials and about 20% of the ethyl benzene asoverhead, removing the remainder of the ethyl benzene and all of thehigher boiling materials as bottoms, and subsequently removing thestyrene from the remainder of the ethyl benzene and higher boilingmaterials by further distillation under a substantial vacuum.

I 2. In the process of separating styrene from a 75 remainder of theethyl 95 through be made thereto.

styrene-containing mixture derived from thedehydrogenation of ethylbenzene to styrene and including benzene, toluene, ethyl benzene andhigher boiling materials, without appreciable polymerization of thestyrene, which comprises effecting an initial distillation underpressure of about mm. Hg absolute, removing toluene, the lower boilingmaterials and at least 20% of the ethyl benzene as overhead, removingthe balance of the ethyl benzene and all of the higher boiling materialsas bottoms; subjecting said overhead to a further distillation undersubstantially atmospheric pressure to separate the ethyl benzene fromthe toluene and lower boiling materials, and subjecting the bottoms fromthe initial distillation of the styrene-containing mixture to a furtherdistillation under a substantial vacuum thereby separating the styrenefrom the remainder of the ethyl benzene and of the higher boilingmaterials.

3. In the separation of a styrene-containing mixture without anyappreciable'polymerization of the styrene because of heat, said mixturebeing derived fnom the dehydrogenation of ethyl benzene to styrene andincluding benzene, toluene, ethyl benzene and higher boiling materials,the steps which comprise effecting the primary distillation of suchmixture under a pressure of approximately 160 mm. Hg absolute toseparate about 20% of the ethyl benzene and nearly all of the tolueneand lower boiling materials as overhead from the majority of the ethylbenzene and all of the higher boiling materials as bottoms, subjectingthe said overhead to a further distillation under substantiallyatmospheric pressure to separate the benzene as overhead from thetoluene and ethyl benzene as bottoms, subjectlng the said bottoms to afurther distillation under substantially atmospheric pressure toseparate thetoluene as overhead and the ethyl benzene as bottoms,distilling the bottoms from the original distillation comprising theremainder of the ethyl benzene and all of the higher boiling materialsunder a substantial vacuum to separate the ethyl benzene and styrene asoverhead from the major portion of the higher boiling materials asbottoms, introducing the ethyl b z styrene overhead into the first stageof a twostage distillation system, each stage of which is maintainedunder a substantial but independent vacuum, whereby the temperature ofdistillation is maintained sufliciently low to prevent unduepolymerization of the styrene, removing ethyl benzene as the overheadfrom this first stage, removing the bottom streams from this first stageand introducing into the top of the second stage to serve as refluxtherein, condensing the overhead vapors from this second stage andrevaporizing the resulting condensate and introducing the resultingvapors into the bottom of the first stage, andremovlng a styreneconcentrate under a substantial vacuum to separate substantially purestyrene as overhead from the remaining high boiling materials asbottoms.

4. In the process of separating styrene from a styrene-containingmixture derived from the dehydrogenation of ethyl benzene to styrene and1 including benzene, toluene,- ethyl benzene and higher. boilingmaterials, without appreciable polymerization of the styrene, whichcomprises effecting an initial distillation under a pressure ofapproximately 160 mm. I-Ig absolute, removing toluene. lower boilingmaterials and about 29% of the ethyl benzene as overhead, removing thebenzene and all of. the

- 2,457,861 9 10 higher boiling materials as bottoms. subjectingREFERENCES CITED said overhead to a further distillation undersubstantially atmospheric pressure to separate the file of this patent;ethyl benzene from the toluene and lower boiling The followingreferences are of record in the v materials, and subjecting the bottomsfrom the 5 UNITED STATES PATENTS initial distillation of thestyrene-containing mix- Nu b Name Date ture to a further distillationunder asubstantial 2,370,948 Gadwa Mar. 6, 1945 venting:1 theielggsefiizrlazing the stiz'ireifietlllfrcirnn 1tlhe OTHER REFERENCES ema er0 e enzen an er {mung materials e y e o e g Mair et al.: 2'! Bureau ofStandards Journal of Research, 58-61 (1941). Copy in Scientific GADWA.Library 202424

